Due to practicaI considerations including safety and efficiency, an antenna is almost always located at a remote location with respect to the transmitter or receiver associated therewith. Thus, in conventional antenna systems, a transmission line of the two conductor type such as a coaxial cable or a twin lead line of substantial length must be used to connect the antenna to the transmitter or receiver.
For optimum performance, the impedance of an antenna must be matched to the impedance of the transmission line and any mismatch in impedance therebetween will increase the standing wave present on the transmission line when transmitting or reduce the signal present on the transmission line when receiving. Such impedance matching must occur physically at the point of interconnection between the transmission line and the antenna and thus at a location remote from the transmitter or receiver.
In the prior art, various devices such as baluns or transformers have been provided at the point of interconnection between the transmission line and the antenna to improve the impedance match therebetween. Such devices have either been highly frequency sensitive or have simply produced a somewhat degraded impedance match over a broader range of frequencies. Attempts to provide an adjustable impedance matching device to enable optimum matching over a range of frequencies have not been satisfactory at least in conventional antenna systems because of the remote physical location at which such a device must be placed.
Conventional antenna systems, to which this invention is applicable, are designed for operation over a range of frequencies which is a significant portion of the nominal operating frequency thereof and include a wide variety of configurations ranging from simple whips and halfwave dipoles through complicated quad antennas and parasitic arrays. This invention is not directly applicable to helical antenna designs and parabolic reflectors which are generally designed for operation over a frequency range which is an insignificant portion of their nominal operating frequencies and usually employ wave guides for coupling between the antenna and the associated transmitter or receiver.
According to the teaching of this invention, a simple, convenient, inexpensive and reliable method and means is provided whereby an optimum impedance match may be obtained between a two conductor transmission line and a conventional antenna at any frequency throughout the frequency range for which the antenna is designed.